Geographically dispersed transceiver sites are often employed in two-way radio communication systems to support subscribers operating over a wide coverage area. Ordinarily, the sites are arranged such that each provides coverage for a particular geographic region, which may have some overlap. A subscriber interfaces with the radio communication system through a particular transceiver site, referred to herein as a server site. When a subscriber moves from one location to another, it may be advantageous to switch server operations from one site to another, such as a neighboring site, in order to maintain a high quality communication link. The process of switching server sites during an ongoing communication session is generally referred to in the art as hand-off or hand-over. The hand-off decision process usually involves a determination of the quality of signals received by the subscriber from its server site, or vice versa, and the availability and suitability of other potential server sites that may be in communication range of the subscriber. The hand-off decision may be made using information gathered by the server site, information gathered by the subscriber, or a combination of both.
In one prior art example, a subscriber maintains a list of control frequencies used by neighboring sites, and monitors the signal quality and/or other communication parameters for signals emanating from these sites. The information gathered by the subscriber is then used to select a suitable hand-off server site. The list of frequencies representing neighboring sites is referred to herein as a monitor list. The monitor list may be preprogrammed into the subscriber or downloaded from the system. A prior art hand-off process is described in U.S. Pat. No. 5,260,943, issued to Comroe et al., on Nov. 9, 1993, for a TDM Hand-off Technique Using Time Differences, the entire contents of which are hereby incorporated by reference. Here, a subscriber receives a monitor list from a site with which it is communicating satisfactorily. This monitor list is used to determine potential server sites for use when a hand-off becomes necessary.
A problem may arise when a subscriber is engaged in an ongoing communication session and performs a hand-off process, thereby switching to a new server site. Potentially, the monitor list could become outdated such the subscriber is unable to switch to an appropriate server site that is not on the list. For example, consider a subscriber that performs a hand-off because of a temporary radio frequency (RF) interference or obstruction, such as from a passing vehicle or from a building or bridge. Until the monitor list is updated, the subscriber may be unable to revert to the old server site that was previously providing system services, as the old server site was not in its monitor list. Such an occurrence is a commonly encountered problem in some prior art systems. Oftentimes, priority is given to utilizing available communication resources to maintain the ongoing communication session without interruption. Thus, the system may be unable to provide new monitor list in a timely manner. The problem becomes more acute when the subscriber is engaged in a group dispatch call along with several other subscribers, and much coordination is needed to provide new monitor list information without losing any of the ongoing communication.
It is desirable to provide a system in which a subscriber is provided with adequate monitor list information to determine potential server sites while participating in an ongoing communication session. Preferably, such information is provided with minimal impact on the ongoing communication session.